This invention relates to a sol-gel technique for producing superconducting oxides, and to thin films of superconducting oxides produced by the technique.
Recently, renewed interest in superconductors was kindled by the discovery of superconductivity at about 30K in compounds in the barium-lanthanum-copper-oxygen system. J. G. Bednorz and K. A. Muller, Z. Phys. B., Vol. 64, No. 189, March 1986. The highest temperature previously observed for the onset of superconductivity had been 23.2K for the intermetallic compound Nb.sub.3 Ge.
Intensive research conducted after the Bednorz-Muller announcement has resulted in dramatic increases in Tc, the critical temperature at which the onset of superconductivity occurs. For example, superconductivity above 90K has very recently been reported in the square planar compound system ABa.sub.2 Cu.sub.3 O.sub.6+x, where A is yttrium or a rare earth, by P. H. Hor et al., Phys. Rev. L., Vol. 58, No. 18, May 4, 1987; and superconductivity at 155K has most recently been reported in a multiphase material of the nominal composition YBa.sub.2 Cu.sub.3 F.sub.2 O.sub.y, by S. R. Ovshinsky et al., Phys. Rev. L., Vol. 58, No. 24, June 15, 1987.
While these dramatic developments presage a host of new applications using these new superconductors, many practical problems remain to be solved before successful commercialization of these applications can be realized.
For example, the new "high temperature" superconductors are essentially ceramic oxides, and their fabrication into certain shapes such as thin films is difficult or impossible to achieve by conventional ceramic processing techniques. Moreover, conventional techniques do not allow the close control over composition, morphology and phase formation which have been found necessary for the successful production of these high temperature superconducting oxides. Thus, the mixing together of starting materials followed by firing the mixture at elevated temperatures to form the superconducting compound by solid state reaction results in a fired composition whose homogeneity will be largely determined by the intimacy of mixing of the starting materials, which is limited by the particle size of these starting materials. Such inhomogeneity, as well as loss of material through volatilization and diffusion of impurities from surrounding structures enabled by the high reaction temperatures, may interfere with the obtaining of the desired superconducting phase.
Furthermore, while solid state reaction usually results in some densification of the particulate starting mixture, final densities near theoretical are rarely obtained, except by resort to extraordinary measures such as hot pressing. High density is thought to be important in superconducting films because it maximizes the number of paths available for conductivity of electrons through the material.
Various techniques other than conventional powder processing techniques have been disclosed for producing mixed oxide ceramics and glasses. For example, in British patent 1,266,494, mixed oxide microspheres are formed by dispersing an aqueous metal salt solution in a non-miscible organic liquid, and adding a base to induce gelling of the dispersed aqueous phase by hydrolysis. The gelled particles are then separated from the organic phase and heated to effect solid state reaction.
In U.S. Pat. No. 4,278,632, silica-titania binary glasses are formed on a substrate by: first partially hydrolyzing one metal alkoxide by adding water to a dilute organic solution of the alkoxide; then reacting this solution with another metal alkoxide solution to polymerize the reactants; then coating the solution on a substrate; drying the coating; and heating the coating at a temperature of about 400 to 1000 degrees C. to convert the coating to a glass. Similar techniques for glass formation are disclosed in U.S. Pat. Nos. 4,472,510; 4,420,517 and 4,419,115. However, none of these patents suggests that their processes can be used to make superconducting materials.
In co-pending U.S. patent application Ser. No. 812,219, filed Febr. 10, 1986 and assigned to the present assignee Ser. No. 812,219, there is described a sol-gel process in which a gellable liquid of partly hydrolyzed alkoxides is applied to a substrate, gelled and heated to form a luminescent thin film. However, such luminescent materials do not require the close control over composition, morphology and phase formation which have been found necessary for the successful production of high temperature superconducting oxides.
Accordingly, it is a principal object of the invention to produce dense, homogeneous thin films of high temperature superconducting oxide compositions.
It is another object of the invention to provide a method for producing superconducting oxides which enables close control over the composition, morphology and phase formation of such films.
It is a still further object of the invention to provide a sol-gel process for producing superconducting oxides.